In English, the definite article, “the,” is used when referring to a person or thing previously mentioned or specifically known from before. For example:
I like the cake you made.
In this example, “the cake” is something that was already made and the speaker and addressee (the maker of the cake) have already talked about. Another example is:
Did you see the weather report for today?
In the above question, “the” is understood to refer to a local weather report and it does not even need to have been mentioned in an earlier conversation.
“The” can be used for both singular and plural nouns, countable and non-countable alike. It is also used before a group, class, nationality, or a species:
The Chinese are making big strides in technological progress.
The dolphin is known for its intelligence.
The definite article also precedes proper nouns that are geographical features (e.g. mountain ranges, rivers, oceans): the Rocky Mountains, the Nile, the Atlantic (Ocean). Single mountains take no article (Mount Whitney). Other proper nouns (not specific people) that take “the” include places and things such as the Metropolitan Museum of Art (the Met), the Washington Monument, the L.A. Times, and the Golden Gate Bridge.
One exception to the rule about people and proper names is when differentiating between two people with identical names. For example, you might know someone named George Clooney (not the famous actor), and mentioning this to someone might elicit the response:
You know the George Clooney?! -No, he is not the famous George Clooney you are thinking of.